1. Field of the Invention
The present invention relates to drilled hole depth control, and more particularly concerns simple, inexpensive measurement of a reference position of the end of a rotating drill bit.
2. Description of Related Art
In a typical automatic drilling or routing machine a drill bit or drill point is inserted into a chuck of a spindle that securely grasps and rotates the bit. The chuck is moved relative to a workpiece in X and Y positions, horizontally, to locate the horizontal position of the bit for proper location of the hole to be drilled. The spindle with its bit is then moved vertically in a Z axis direction to cause the rotating bit to penetrate a workpiece to a predetermined depth so as to drill a hole to a predetermined depth. Depth of the hole is controlled by the machine tool controller, which calculates and commands all of the tool positioning, including the Z axis positioning for hole depth drilling. Although the machine tool control can precisely command the magnitude of Z axis motion, precision of hole depth control requires precision information concerning the Z axis position of the bit end. As each bit is inserted into a spindle it may occupy different longitudinal positions relative to the spindle, depending upon how far into the spindle the bit is inserted. Even if the position of the bit relative to the spindle be precisely known, it is still necessary to calibrate the system to precisely determine the height of the bit end at some given Z axis position of the machine tool itself.
In a typical drill bit locating system of the prior art a pressure plate system is employed in which the tip of the drill bit is caused to contact the pressure plate. When sufficient pressure is applied to the plate by the drill bit a signalling circuit is activated that enables the machine to know and store the Z axis position of the drill tip. Tips of drills, particularly with very small diameter drills, are subject to significant abuse in the practice of this method, because the pressure contact will dull the tip and shorten its life span. Moreover, accidentally applied excess pressure may break fragile small diameter drill bits.
Other systems have employed a very fine diameter laser beam that is directed at the tip of the drill bit to identify drill bit Z axis position with good inherent accuracy. However, such a laser system is excessively expensive.
Accordingly, it is an object of the present invention to provide a drill bit end position sensor that avoids or eliminates above mentioned problems.